By all means prune it back...it will spring back even stronger in the spring. Usually in our zone the advice is to prune back in very early spring, but I have pruned a few well-established clematis back late in the fall with no ill effects. If your clematis is a type III (flowers on new growth)it should be cut back to 6-12 inches from the ground anyway. If it's a type II (flowers on old AND then new growth), then you will lose the early (May) flowers, and just get blooms in late summer. Type I (flowers only on old growth from last year) you'll have to wait until the next spring to see flowers.
It's really amazing how fast clematis can grow once spring arrives. I also find it hard to keep ahead of mine...you have to get out there every day if you want to train in any way. The best thing would be a trellis you could leave in place all winter - a necessity if your plant turns out to be type II or type I - since you want to save as much old wood as possible and leave them attached to the support over winter. They will then just require a little clean up of dead tips and re-attaching stray vines in the spring.

Hope this helps.

Judith

Thank you so much Judith. We were very worried about this. It is a good idea to have a permanent trellis. I will let my mother know. It's reassuring to know that others other than us sometimes having trouble with keeping up wih clematis growth. I appreciate your speediness as well. thank you. Now I'm not so worried about losing the plant over the winter and am happy to know that it will indeed grow back healthier. Than you so much for the third time and enjoy the holidays :)

Some Clematis grow too big too grow on a mailbox post even if you do prune them to the ground every year.

Pruning won't hurt it. Whack away. After your frosts have started so it won't put out new growth now.